1. Field of the Invention
The present invention relates to an engine control device and an engine control method for a straddle type vehicle, and more specifically to an engine control device and an engine control method having engine output decreasing means for decreasing engine output when a failure is detected in a throttle valve control system.
2. Description of Related Art
An electronic throttle valve system for electronically controlling the opening of a throttle valve can control the amount of intake air to an engine accurately. Thus, the electronic throttle valve system can reduce emission and fuel consumption and has been used in some four-wheeled motor vehicles.
Such an electronic throttle valve system is equipped with a function that stops the actuation of the throttle valve by an electric motor and returns the throttle valve in the closing direction by the urging force of a spring when a throttle valve control system has a failure.
The speed at which the throttle valve is rotated in the closing direction by the urging force of the spring is very high. Thus, the output of the engine is rapidly decreased. In the case of a four-wheeled passenger vehicle, a driver does not feel a change in the behavior of the vehicle even when the engine output is rapidly decreased since the vehicle is heavy. In the case of a two-wheeled motor vehicle which is light, however, a rapid decrease in the engine output gives the rider an uncomfortable feeling.
Japanese Patent Document JP-A-Hei 6-248979 discloses a method of closing a throttle valve gently by applying a resistance to the rotation of the throttle valve, which is urged in the closing direction by a spring, to prevent rapid rotation of the throttle valve. A rapid decrease in the engine output can be thus avoided and the vehicle does not make jerky movements even when running in low gear.
Japanese Patent Document JP-A-2003-65140 discloses a method of decreasing engine output gradually by controlling ignition to the engine, as well as by controlling the throttle valve as in JP-A-Hei 6-248979. More specifically, retarding ignition timing, decreasing the frequency of ignition, or cutting off ignition is conducted to decrease engine output smoothly.
It is certain that closing the throttle valve gently can avoid a rapid decrease in the engine output. However, there might be a range of the opening of the throttle valve in which the engine output does not decrease. In this case, since the engine output is constant, the rider might feel an acceleration feeling.
Meanwhile, in the case of controlling ignition to the engine to control the engine output, more responsive control of the engine output is enabled compared to the case in which the rotation of the throttle valve is controlled, so that the engine output is decreased invariably. The rider thus does not feel any acceleration feeling as described above.
However, retarding ignition timing, decreasing the frequency of ignition, or cutting off ignition is conducted under preset conditions. Thus, the way the engine output decreases varies depending on the vehicle's operating conditions including engine speed in the event of a failure, and is not uniquely defined. As a result, the engine output decreases but the rider might get an uncomfortable feeling.